Process for producing polystyrene foam

ABSTRACT

The present invention relates to a process to produce polystyrene foam comprising mixing a predetermined amount of masterbatch mix and a styrenic polymer, heating this mixture, injecting CO 2  and/or nitrogen, in gas or liquid form, as a blowing agent and subsequently extruding, cooling and drying the resulting polystyrene foam.

This is a divisional of application Ser. No. 08/184,073, filed Jan. 18,1994, now U.S. Pat. No. 5,342,857, which in turn is a divisional ofapplication Ser. No. 08/061,670, filed May 14, 1993, now U.S. Pat. No.5,302,624, which in turn is a divisional of application Ser. No.07/891,866, filed Jun. 1, 1992, now U.S. Pat. No. 5,210,006.

FIELD OF THE INVENTION

This invention relates to a process for producing polystyrene foamutilizing a predetermined amount of masterbatch mix.

BACKGROUND OF THE INVENTION

Polystyrene foam, particularly in sheet form, is presently being madefrom a number of blowing agents which have many undesirablecharacteristics. Volatility, flammability, poor thermoforming qualities,brittle physical properties, high cost, or an adverse affect to theozone layer are just a few. Examples of the blowing agents that producethese characteristics in the production of polystyrene foam wouldinclude aliphatic hydrocarbons and fully or partially halogenatedhydrocarbon.

A variety of normally gaseous or liquid blowing agents have beenproposed for olefinic or styrenic polymers, including virtually all ofthe common atmospheric gases and lower hydrocarbons.

For polystyrene, the C₄ -C₆ alkanes have gathered widespread acceptance,especially pentane. Following a typical extrusion foaming step, thestock material is ordinarily aged before thermoforming into containersor the like. During aging, the foam cells and polymeric matrix becomepartially depleted of volatile hydrocarbons, which enter the atmosphere.However, potential atmospheric contamination by these by-products offoam manufacture has led workers to seek non-polluting alternativeblowing agents, such as the usual atmospheric gases, e.g., nitrogen,carbon dioxide.

U.S. Pat. Nos. 4,344,710 and 4,424,287 disclose blowing agents which areblends of carbon dioxide and aliphatic or fully or partially halogenatedhydrocarbons. These patents, however, state that the use of 100% carbondioxide as a blowing agent has not been successfully employed due to theextreme volatility. Use of these materials is said to producecorrugation and surface defects.

Others have developed methods and apparatuses for injecting a gaseousblowing agent into a molten resin charge, such as polystyrene in anaccurate and precise manner. One example of this can be seen in U.S.Pat. No. 4,470,938.

Numerous experiments have been aimed at first preparing a masterbatchmix of a plasticized blowing agent and nucleator in which the blowingagent is uniformly distributed, and subsequently this premixture isadded to polystyrene which is then extruded into foam. However, it hasbeen shown that it is not possible to mix polystyrene with such ablowing agent without premixture foaming, since the blowing agentdecomposes at the melt temperature of the polystyrene. This is anundesirable effect.

U.S. Pat. No. 4,940,735 teaches preparation of a masterbatch containing30 to 80 weight percent of a plasticizer, 20 to 70 weight percent ofblowing agent, and 10 to 20 weight percent of a cell regulator. Whilethis masterbatch mix is an improvement over earlier art, it does nothave all of the advantages of the present invention.

Thus, the object of the present invention was to develop a masterbatchmix for the preparation of polystyrene foams which when used platicizes,improves impact, improves melt flow and strength, and nucleates theinert gases in polystyrene in order to shorten extrusion process time.Also, the masterbatch mix must be utilized with CO₂ or Nitrogen as theblowing agent. Moreover, the polystyrene foam produced have very finecells and low densities, thus, thermoforming is easier.

The present invention relates to a process for producing polystyrenefoam, said foam being prepared by extruding a styrenic polymer and apredetermined amount of masterbatch mix in conjunction with carbondioxide, nitrogen or mixtures thereof as a blowing agent. Themasterbatch plasticizes, improves flow rate, melt strength and nucleatesthe inert gases in the polystyrene foam which in turn shortens processtime. The resulting polystyrene foam has a substantially uniform cellwall thickness and improved polymer melt strength and improved density.

DETAILED DESCRIPTION OF THE INVENTION

The masterbatch mix is a plasticizer which improves the flowcharacteristics of the foam. This masterbatch mix comprises essentiallyabout 1 to 20 weight percent of stoichiometric amounts of monosodiumcitrate and sodium bicarbonate encapsulated in vegetable oil, about 3 to50 weight percent of styrene-ethylene/butylene-styrene block copolymer,about 20 to 80 weight percent of alpha methyl styrene, and about 1 to 20weight percent of white mineral oil. Also present is about 0.2 weightpercent of silica which is not a required element of the masterbatchmix, but aides in maintaining the free flow capability of themasterbatch mix under long term storage conditions. The masterbatch mixalso increases the amount of inert gas concentration. Inert gases suchas CO₂ and/or Nitrogen have no plasticization capabilities and leave thepolystyrene foam in a non-plasticized state. The masterbatch mixnucleates and chemically plasticizes the foam so that it is moreflexible.

An essential element of the masterbatch is the alpha methyl styrene. Allcommercially available styrene polymers can be used as the polystyrenecompound. However, it is essential that the Vicat softening temperatureof the chosen styrene polymer be between 55 and 65 at 50° C./hr. Onecommercially known is Amoco's Resin 18-240 which has a Vicat softeningtemperature of 60.5 at 50° C./hr and 62.0 at 120° C./hr.

Another essential element of the masterbatch is thestyrene-ethylene/butylene-styrene block copolymer. One commerciallyknown is Shell's Kratron G 1650.

The blowing agent preferably used is that grade of carbon dioxide ornitrogen which is normally commercially available. For the purposes ofthis specification such carbon dioxide or nitrogen is referred to as 100percent carbon dioxide or nitrogen even though they may contain minorimpurities or amounts of other compounds.

The blowing agent can be added to a polymer melt in either a gaseous orliquid form or combinations thereof.

The use of an extrusion process for the manufacture of polystyrene foamis typical, but is not required. Such a process includes a primaryextruder, a blowing agent addition system, a secondary extruder, anannular die, a sheet cutter or slitter and a sheet gathering device.However, the use of this exact equipment set up is not required in theprocess of this invention.

Polystyrene foam is formed in a continuous process by delivering awell-mixed and uniform blend of styrenic polymer and masterbatch mix tothe extruder throat. Masterbatch mix is preferably about 0.001 to 0.035weight of the total mixture. Once in the screw, while being rotated at acontrolled RPM, the blend of styrenic polymer and masterbatch mix, orfeed, is heated to a temperature above the melting point of the blend,about 250° to 500° F. It is then delivered with the use of relativelystable pressure in the range of about 4000-6000 psi, to the point ofinjection. Here, an injection system delivers carbon dioxide in gas orliquid form and/or nitrogen in gas form, or combinations thereof, intothe melted feed.

Next, the blowing agent, or carbon dioxide and/or nitrogen, and meltedfeed pass into a second extruder. This extruder is designed for maximumcooling capability. It is of much larger capacity than the first screw.In this screw, a minimum of shear is desired. It is achieved by keepingthe screw's roof diameter constant. The molten feed and blowing agentare cooled to a temperature below the first temperature and above themelting temperature of the feed.

The feed exits this screw through a die at a temperature between about250°-270° F. and a pressure of about 2,500-3,750 psi. The exitingmaterial is stretched out over a cooling drum and drawn to the desireddensity. The polystyrene foam sheet is then slit and can be wound intolarge rolls.

EXAMPLE 1

The following were continuously fed into a mixer:

54.8% of equimolar amounts of monosodium citrate and sodium bicarbonateencapsulated in vegetable oil

30.5% of poly alpha methyl styrene

12.0% of styrene-ethylene/butylene-styrene block copolymer

7.5% of white mineral oil

0.2% silica

which resulted in a masterbatch mix that was then fed into an extruder.

The masterbatch was melted at a temperature of 310° to 550° F. and CO₂in a liquid form was injected as a blowing agent. The resulting mixturewas subsequently extracted, cooled and dried. The Vicat softeningtemperature of the poly alpha methyl styrene was 60.5 (at 50° C./hr).

The resulting product had a density of 0.374 FTLB/in, a tensile strengthof 194.3 psi, and a low cell density which resulted in easierthermoforming.

EXAMPLE 2

By feeding the following continuously into a mixer:

19.8% of equimolar amounts of monosodium citrate and sodium bicarbonateencapsulated in vegetable oil

67.5% of poly alpha methyl styrene

10.0% of styrene-ethylene/butylene-styrene block copolymer

2.5% of white mineral oil

0.2% silica

a masterbatch mix is obtained that can be fed into an extruder.

Following the same procedure as Example I, the resulting product wouldhave similar characteristics.

EXAMPLE 3

By feeding the following continuously into a mixer:

38.8% of equimolar amounts of monosodium citrate and sodium bicarbonateencapsulated in vegetable oil

36.6% of poly alpha methyl styrene

14.4% of styrene-ethylene/butylene-styrene block copolymer

9.0% Of white mineral oil

0.2% silica

a masterbatch mix is obtained that can be fed into an extruder.

Following the same procedure as Example I, the resulting product wouldhave similar characteristics.

The present invention has been described with preferred embodiments. Itis to be understood however that modifications and variations may beresorted to, without departing from the spirit and scope of theinvention, as those skilled in the art would readily understand. Thesemodifications and variations are considered to be within the scope ofthe appended claims.

What is claimed is:
 1. A polystyrene foam prepared according to aprocess comprising:(A) feeding a predetermined amount of masterbatch mixand styrenic polymer to a first zone and heating said mixture to a firsttemperature above the melting point of said mixture to form a moltenmixture wherein said masterbatch mix comprises: (1) monosodium citrateand sodium bicarbonate encapsulated in vegetable oil; (2)styrene-ethylene/butylene-styrene block copolymer; (3) poly alpha methylstyrene; (4) white mineral oil; and (5) silica; (B) passing said moltenmixture through an injection zone wherein a gas such as CO₂ and/ornitrogen or mixtures thereof, in either gas or liquid form is injectedinto said molten mixture; (C) feeding said molten mixture and CO₂ and/ornitrogen or mixture thereof, in either gas or liquid form, into a secondzone and cooling the molten mixture and CO₂ and/or nitrogen to a secondtemperature below the first temperature and above the meltingtemperature of the mixture; and (D) passing said mixture and CO₂ and/ornitrogen or mixtures thereof, through die means.
 2. A polystyrene foamprepared according to a process comprising:(A) feeding a predeterminedamount of masterbatch mix and styrenic polymer to a first zone andheating said mixture to a first temperature above the melting point ofsaid mixture to form a molten mixture wherein said masterbatch mixcomprises: (1) from 1 to 70 weight percent of stoichiometric amounts ofmonosodium citrate and sodium bicarbonate encapsulated in vegetable oil;(2) from 3 to about 50 weight percent ofstyrene-ethylene/butylene-styrene block copolymer; (3) from 20 to 80weight percent of poly alpha methyl styrene; (4) from 1 to 20 weightpercent of white mineral oil; and (5) from 0.1 to 0.3 weight percent ofsilica; (B) passing said molten mixture through an injection zonewherein a gas such as CO₂ and/or nitrogen or mixtures thereof, in eithergas or liquid form is injected into said molten mixture; (C) feedingsaid molten mixture and CO₂ and/or nitrogen or mixture thereof, ineither gas or liquid form, into a second zone and cooling the moltenmixture and CO₂ and/or nitrogen to a second temperature below the firsttemperature and above the melting temperature of the mixture; and (D)passing said mixture and CO₂ and/or nitrogen or mixtures thereof,through die means.
 3. A polystyrene foam according to claim 2, whereinsaid masterbatch mix comprises:(1) 54.8% of equimolar amounts ofmonosodium citrate and sodium bicarbonate encapsulated in vegetable oil;(2) 30.5% of poly alpha methyl styrene; (3) 12.0% ofstyrene-ethylene/butylene-styrene block copolymer; (4) 7.5% of whitemineral oil; and (5) 0.2% silica.
 4. A polystyrene foam according toclaim 2, wherein said masterbatch mix comprises:(1) 19.8% of equimolaramounts of monosodium citrate and sodium bicarbonate encapsulated invegetable oil; (2) 67.5% of poly alpha methyl styrene; (3) 10.0% ofstyrene-ethylene/butylene-styrene block copolymer; (4) 2.5% of whitemineral oil; and (5) 0.2% silica.
 5. A polystyrene foam according toclaim 2, wherein said masterbatch mix comprises:(1) 38.8% of equimolaramounts of monosodium citrate and sodium bicarbonate encapsulated invegetable oil; (2) 36.6% of poly alpha methyl styrene; (3) 14.4% ofstyrene-ethylene/butylene-styrene block copolymer; (4) 9.0% of whitemineral oil; and (5) 0.2% silica.
 6. A polystyrene foam comprising astyrenic polymer matrix, between about 0.0002-0.028 weight percent alphamethyl polystyrene 0.00001 to 0.0105 weight percent of silica andbetween about 0.00003-0.0175 weight percentstyrene-ethylene/butylene-styrene block copolymer, said foam having acell structure wherein the cells contain a gas comprised of carbondioxide, nitrogen or mixtures thereof.